1. Field of the Invention
This invention relates to arc spray guns of the type in which the ends of two metal wires are melted in an electric arc and the resulting molten metal sprayed on a workpiece to be coated. The invention relates more particularly to arc spray guns adapted for utilization with two metal wires of different cross-sectional area and of relatively hard metals.
2. Prior Art
In arc spray guns, two metal wires are connected to respective terminals of an electrical power source and the ends brought together to form an electric arc which causes melting of the wires. A jet of air or other gas applied from a nozzle in proximity to the zone of melting, sprays the molten metal thus produced on a workpiece to be coated. The wires are advanced into the melting zone to maintain the arc and replace wire previously melted and sprayed.
While arc spray guns can operate on either AC or DC, the latter is much preferred as it results in a much quieter and more stable arc. Unfortunately, DC operation also has an attedant disadvantage: the positive electrode is heated more than the negative electrode by the electric arc; consequently, the positive wire is consumed at a higher rate and, assuming the wires are fed at the same rate, the arc becomes asymmetrical with respect to the gas jet which sprays the molten metal. To cope with this problem, it has been proposed in British Pat. Specification No. 1,357,582 that the positive wire be advanced at a rate sufficiently higher than the negative wire to compensate for the difference in the rate of consumption. The same patent specification alternatively proposes the use of wires of dissimilar cross-section, the positive wire being the larger, of course. The U.K. Specification states that the use of wires of different diameters would be "operationally disadvantageous" but is has been discovered that the problems attendant to the utilization of dual wire sizes in an arc spray gun are, in fact, susceptible of solution so as to enable the utilization of this concept in a spray gun of simple, practical, and efficient design.
One of the major problems encountered in employing dual wire sizes in a spray gun is the difference in the flexibility of the wires stemming from the differences in cross-section. This problem, though encountered when spraying soft metals such as aluminum and zinc, becomes particularly acute when handling less malleable metals such as steel.
In conventional arc spray guns, the wires are guided to the arc location by threading them through a pair of parallel guide members and then through a pair of tubular electodes converging toward the melting zone. The electrodes are connected to respective terminals of the power source and electrical contact to the wires results from the fact that the wires must bend when passing from the parallel guide members to the converging electrodes, this bending action being accomplished and accompanied by rubbing contact between the inner walls of the electrodes and the wires passing therethrough. If dual wire sizes are introduced into conventional spray guns, particularly wires of steel or other low malleability metals, the thicker and therefore, more rigid wire resists the necessary bending as it enters the electrode causing feeding difficulties which may take the form of either or both jamming of the gun and slippage of the wire feeding mechanism. Even in marginal situations where it is possible to achieve operation, the thicker wire imposes undue strains and wear on the feeding and guiding mechanisms of the gun.